The principle of coupled resonators in an NMR radio-frequency coil is well known in the art. See, for example, "A Dual-Tuned Probe and Multiband Receiver Front End for X-Nucleus Spectroscopy with Proton Scout Imaging in Vivo" by James Tropp and Satoshi Sugiura, Magnetic Resonance in Medicine, 11 (1989), pp 405-412 (hereinafter "Tropp"). In Tropp, two separate loop coils are coupled, primarily capacitively, to produce a two loop coupled resonator. See, for example, FIG. 3 thereof.
In "A 4-Ring, High B.sub.1 Homogeneity, High Sensitivity Probe Whose Signal is Invariant to Changing Sample Characteristics" by David I. Hoult and Roxanne Deslauriers, Abstracts of 31st ENC, p. 51 (1990), (hereinafter: "Hoult") a 4-loop device is disclosed. The 4-loops are coupled inductively to produce a 4-loop coupled resonator. However, all of the loops are substantially parallel to one another and in order for the device to be suitable for head imaging, it is desired that the device produce a field which is transverse to the main magnetic field. In the event the main magnetic field is oriented along the long axis of the subject, as is generally the case in NMR imaging, the head must be inserted through the coils substantially in the x-z plane (as shown in the Abstract) rendering the device unsuitable for head imaging.
In general, an NMR radio-frequency coil suitable for head imaging must solve the problem of achieving high signal to noise ratio in conjunction with good uniformity. In general, the signal to noise ratio of a coil may be increased by decreasing its size, thereby improving its filling factor. However, the filling factor can be increased only until either a barrier of patient discomfort or a barrier of unacceptable non-uniformities in the radio-frequency field of the coil is reached.